Chemical Symbols & Formulae

Step 1: Identify the Ions: Determine the symbols and charges for the cation (usually the metal) and the anion (usually the non-metal).

Step 2: The Cross-over Method: Take the numerical value of the charge of the first ion and make it the subscript of the second ion. Take the numerical value of the charge of the second ion and make it the subscript of the first ion.

Step 3: Simplify the Ratio: If the subscripts have a common factor, divide them by that factor to reach the simplest whole-number ratio (empirical formula).

Step 4: Handle Polyatomic Ions: If a formula requires multiple units of a polyatomic ion (e.g., nitrate or sulfate), place the entire ion inside brackets before adding the subscript outside.

Binary Compounds: For compounds containing only two elements, the name of the second element usually ends in the suffix -ide (e.g., Oxide, Chloride).

Polyatomic Compounds: If the compound contains oxygen along with another non-metal, the suffix often changes to -ate (e.g., Sulfate, Carbonate) to indicate the presence of the oxygen-containing complex ion.

Metal Valency: For transition metals that can form multiple ions, the charge is indicated using Roman numerals in brackets, such as Iron(II) vs. Iron(III).

Check Capitalization: Always verify that the second letter of a symbol is lowercase. Writing 'NA' instead of 'Na' is a common error that can lead to marks being lost.

The Bracket Rule: Only use brackets for polyatomic ions when the subscript is 2 or greater. For example, use $NaOH$, not $Na(OH)$, but use $Mg(OH)_2$.

Neutrality Check: After writing a formula, multiply the number of atoms by their respective charges. The sum must equal zero for a neutral compound.

Simplest Ratio: For ionic compounds, always ensure the subscripts are in their lowest possible ratio. A formula like $Ca_2O_2$ should always be simplified to $CaO$.